Among the classes of so-called "frequency independent" antennas are the equiangular antennas and the log-periodic antennas. Log-periodic antennas are so termed because any portion of the structure may be scaled so that the electrical properties repeat periodically with the logarithm of the frequency. In principle, such antennas may be arranged to have any desired bandwidth, but in practice the bandwidth is limited by the manufacturing tolerances possible at the high frequency end, and the low frequency is ordinarily limited by the space required for the low-frequency antenna elements Frequency-independent and log- periodic antennas are well known in the art and are described, for example, in the text "Antenna Engineering Handbook" edited by Jasik, published by McGraw-Hill.
A particular type of log-periodic antenna is described in U.S. Pat. No. 3,210,767 issued Oct. 5, 1965 to Isbell. The Isbell antenna is a planar (all dipole elements lying substantially in one plane) log periodic including a number of bays of half-wave dipoles fed by what amounts to an elongated balanced two-wire or two-conductor transmission line. The lengths of the dipole elements taper from a maximum at the low-frequency end to a minimum at the high-frequency or "feed" end.
Those skilled in the art know that antennas are reciprocal passive devices in which various properties are identical in both the transmitting and receiving modes. For example, the directivity and beamwidth are identical in both transmitting and receiving modes of operation. Ordinarily, description of antenna operation is couched in terms of either transmission or reception, the other operation being understood.
When the feed transmission line of the Isbell antenna is fed with signal at a frequency near the center of the operating frequency band from the side of the transmission line having the relatively smaller dipole elements, the signal propagates along the transmission line. When propagating past the relatively small dipole elements near the feed point, the signal "sees" the dipole elements as relatively small capacitances which shunt the effective capacitance of the transmission line. The small radiating elements have relatively small radiation resistance in series with the relatively large reactance of the equivalent capacitance, and therefore radiate very little energy. Thus, the signal effectively propagates along the transmission line unaffected by the small dipole elements. Eventually, the signal reaches regions in which the dipole elements coupled to the transmission line have lengths of approximately .lambda./4 (.lambda./2 for the entire dipole). In these regions, the propagating signal "sees" real dipole impedances or radiation resistances coupled across the impedance of the transmission line. The dipole impedances are of the same order of magnitude as the characteristic impedance of the transmission line. Consequently, at frequencies at which the dipole elements are approximately .lambda./2 long, energy is coupled from the transmission line to the elements and radiated thereby. The log periodic dipole array is arranged so that more than one dipole receives significant energy at any midband operating frequency, so that an array of elements is formed for radiation at that frequency. The arraying of the elements and their relative phases results in radiation back toward the feed. Thus, a radiated beam is formed in the direction in which the array "points", viewing the array as a whole as an arrowhead pointing in a given direction. If energy were to propagate past the region in which the dipoles are about .lambda./2 long, it would encounter dipoles which approach lengths at which they individually produce multiple-lobed patterns and have impedances which couple energy from the transmission lines. However, most of the signal energy applied at the feed point is coupled out within the .lambda./2 dipole region, so little energy remains to flow to the relatively large dipoles, the radiation of which might perturb the desired antenna radiation pattern.
As so far described, the Isbell log periodic dipole produces a singly polarized signal. Antennas of the general type described by Isbell have been used for the horizontally polarized television receiving antennas, for broadband communication and the like. U.S. Pat. Application Ser. No. 06/936,499 filed Dec. 1, 1986 in the name of Balcewicz describes the simultaneous use of two orthogonal linear polarizations for communication between widely spaced Earth stations. As mentioned in U.S. Pat. No. 4,590,480 issued May 20, 1986 in the name of Nikolayuk et al, singly-polarized or horizontally-polarized signals may not be optimum under all circumstances for television purposes. As mentioned therein, attention has been directed to the broadcasting of circularly polarized signals from a television transmitter in order to reduce the effects of ghosting and to provide uniformity of coverage. Orthogonally crossed log periodic dipole arrays as described in the article "Space Antenna Selection and Design" by Brown et al, published in the Oct. 1965 issue of Systems Design magazine, have long been known to be useful for simultaneous orthogonal linear polarization or, in conjunction with couplers for providing a quadrature phase shift, for transducing circularly polarized or elliptically polarized signals.
The crossed log periodic dipole array antenna when fully deployed, as illustrated in the Brown et al article, includes a transmission line arrangement having an axis which lies parallel to the direction of electromagnetic propagation and also includes two mutually orthogonal .lambda./2 dipole antennas at each of multiple bays. The dipole antennas at one end of the array have lengths of about .lambda./2 at the highest frequency of operation, and at the other end of the array have lengths of .lambda./2 at the low frequency of the operating frequency band. Such an arrangement when in its deployed state may be difficult to mount in position. For example, for VHS television purposes in the United States, each of the two crossed dipoles at the low frequency end of the log periodic array may be ten or more feet long, and when one of the dipoles is horizontal, the other is vertical. Such a structure is very awkward to store or manipulate. It is known to hinge each dipole element near its juncture with the transmission line in order to ease the storage problem. However, the problem of awkwardness in handling reappears once it is deployed ready for mounting. An automatic arrangement for deploying an antenna element is desirable, and especially one which is suitable for deploying the elements of a crossed log periodic dipole array.